Spells are words that when said in a certain order can warp reality to some extent. Mortals cannot use spells due to a lack of magic so only beings such as witches and/or wizards can cast them.
Potential and EffectsEdit
Spells can produce a variety of effects, from conjuring something from thin air, or changing one thing into something else completely different. Spells are supposed to be more powerful when they are spoken with more witches/wizards. Some spells can't even be said without more than one witch involved.
On the show Charmed, if spells are read backwards then they will be undone, rendering them useless.